Human Ancestors: Gatherers or Hunters?

Chimpanzees crave roots and tubers even when food is plentiful above ground, according to a new study that raises questions about the relative importance of meat for brain evolution.

In a study that appeared online the week of Nov. 12 in the early edition of the Proceedings of the National Academy of Sciences, USC College doctoral graduate Adriana Hernandez-Aguilar and her colleagues document a novel use of tools by chimps to dig for tubers and roots in the savanna woodlands of western Tanzania.

The chimps’ eagerness for buried treats offers new insights in an ongoing debate about the role of meat versus potato-like foods in the diet of our hominid ancestors, said first author Hernandez-Aguilar, who collected the field data for her doctoral research at USC.

The debate centers on the diet followed by early hominids as their brain and body size slowly increased toward a human level. Was it meat and potatoes or potatoes and meat?

“Some researchers have suggested that what made us human was actually the tubers,” Hernandez-Aguilar said.

Anthropologists had speculated that roots and tubers were mere fallback foods for hominids trying to survive the harsh dry season in the savanna 3.5 million years ago and later (hominids are known to have consumed meat at least as early as 2.5 million years ago).

But the study found that modern chimps only dig for roots during the rainy season, when other food sources abound.

The finding suggests but does not prove that hominids behaved the same way. Researchers view modern chimps as proxies for hominids because of similarities in habitat, brain mass and body size.

“We look at chimps for the way that we could have behaved when our ancestors were chimp-like,” Hernandez-Aguilar said.

Co-author Travis Pickering of the University of Wisconsin-Madison said: “Savanna chimps, we would contend, are dealing with environmental constraints and problems — evolutionary pressures — that our earliest relatives would have dealt with as well.”

The tuber-digging chimps “suggest that underground resources were within reach of our ancestors,” added co-author James Moore of UC San Diego.

The study was based on observation of 11 digging sites in the Ugalla savanna woodland of western Tanzania.

Chimpanzees were linked to the excavated tubers and roots through knuckle prints, feces and spit-out wads of fibers from those underground foods.

Seven tools were found at three of the sites, with worn edges and dirt marking implying their use as digging implements.

Because chimpanzees in the area are not habituated to humans, Hernandez-Aguilar was unable to observe them directly. She plans to conduct further observations in the area and to advocate for greater protection for the savanna chimps.

“Chimpanzees in savannas have not been considered a priority in conservation plans because they live in low densities compared to chimps in forests,” she said.

“We hope that discoveries such as this will show the value of conserving the savanna populations.”